Synthesis gas, also termed syngas, may be generated by a gasification of carbonaceous feedstocks such as coal, petroleum coke or other carbon-rich feedstocks using oxygen or air and steam at elevated temperature and pressure.
For the production of methanol or hydrocarbons, the desired stoichiometry ratio, R, which refers to the ratio of molar concentrations of the gas components, [R═(H2—CO2)/(CO+CO2)], is preferably in the range 1.4 to 2.5. For generating synthetic natural gas (SNG) the range is preferably in the range 2.8 to 3.3. Other processes (e.g. ammonia production, extraction of hydrogen for use in fuel cells or in a gas turbine) require maximising the yield of hydrogen. To achieve this, it is necessary to subject the raw synthesis gas to the water-gas-shift reaction by passing it, in the presence of steam, over a suitable water gas shift catalyst at elevated temperature and pressure. The CO2 that is formed is then removed in a downstream gas washing unit to give the desired R ratio or hydrogen rich product gas. The synthesis gas generally contains one or more sulphur compounds and so must be processed using sulphur-resistant catalysts, known as “sour shift” catalysts. The reaction may be depicted as follows;H2O+COH2+CO2 
This reaction is exothermic, and conventionally it has been allowed to run adiabatically, i.e. without applied cooling, with control of the exit temperature governed by feed gas inlet temperature, composition and by by-passing some of the synthesis gas around the reactor.
Side reactions can occur, particularly methanation, which is usually undesirable. To avoid this, the shift reaction requires considerable amounts of steam to be added to ensure the desired synthesis gas composition is obtained with minimum formation of additional methane. The costs of generating steam can be considerable and therefore there is a desire to reduce this where possible.